1. Field of the Invention
This invention relates to a heating-fixing device having a heating source emitting a heat radiation, and particularly to a heating-fixing device having the function of preventing overheating of the heating source.
2. Description of the Prior Art
Heating-fixing devices of this type have heretofore been often adopted in image formation apparatuses using the electrophotographic method, the electrostatic recording method, the magnetic photographic method or the like.
In the heating-fixing devices according to the prior art, the surface temperature of a fixing or heating roller heated by a halogen heater or the like as a heating source is detected by temperature detecting means disposed in contact with or in proximity to the fixing roller and a switch inserted in the heater circuit is opened by a temperature control circuit which has received the detection signal, whereby the surface temperature of the fixing roller is controlled so as to be maintained within an allowable range.
However, in case the opening of the switch is not effected due to failure or the like of the temperature control circuit, the fixing roller may be heated to an abnormally high temperature. Therefore, to prevent such excessive temperature rise, a temperature fuse as a heat-responsive member is provided in proximity to the fixing roller, this temperature fuse is series-inserted in the heater circuit and the temperature fuse is fused by receiving the radiant heat from the fixing roller, thereby preventing excessive temperature rise of the fixing roller.
In this case, it is for the following reason that the temperature fuse is disposed in proximity to the fixing roller with a predetermined distance maintained therefrom as described above. Between the fixing roller and the temperature fuse, there is a regular temperature relation depending on the distance. For example, if the surface temperature of the fixing roller is 200.degree. C., then the temperature at a distance of 1 mm from the fixing roller is 180.degree. C., the temperature at a distance of 2 mm from the fixing roller is 160.degree. C., the temperature at a distance of 3 mm from the fixing roller is 130.degree. C., and the temperature at a distance of 4 mm from the fixing roller is 90.degree. C. Accordingly, if a temperature fuse of rated temperature (the temperature at which fusing occurs) 168.degree. C. is disposed, for example, at a distance of 2 mm from the fixing roller, it does not fuse when the surface temperature of the fixing roller is controlled normally within an allowed range, and during abnormality, namely, due to failure or the like of the temperature control circuit, the temperature of the fixing roller rises excessively and the temperature at the position whereat the temperature fuse is disposed reaches 168.degree. C. or higher, whereby the temperature fuse fuses and the supply of power to the heating source can be cut off reliably. However, if the temperature fuse is provided at a position proximate to the fixing roller as described above but spaced apart from the peripheral surface of the fixing roller, the temperature fuse is subject to the influence of the air stream around the roller and the fuse and there may occur the inconvenience that the fuse does not fuse in spite of the fixing roller having actually reached a dangerous temperature, and the amount of heat radiation which the temperature fuse receives from the fixing roller considerably differs if the distance from the roller slightly differs, and thus high accuracy of the position at which the fuse is disposed has been required. Further, with only the radiant heat from the roller, where a temperature fuse which directly controls a heavy current is employed, its heat capacity is great and the time from when the roller has reached an abnormal temperature until the fuse is fused, namely, the response time, is long, and this has led to the problem that temperature rise of the roller progresses in the meantime, and this problem has been particularly serious to a roller of small wall thickness and small heat capacity.